1. Field
The following description relates generally to a vertical-cavity surface emitting laser (hereinafter referred to as VCSEL) driver of a high-speed optical communication system, and more particularly to a VCSEL driver that electrically drives a VCSEL, which is a laser diode for short-haul single mode optical link, or high-density optical link used at a transmitting end of an optical transceiver.
2. Description of the Related Art
An optical transceiver includes an optical transmitter and an optical receiver. A laser diode (LD) is a main optical element of an optical transmitter, and a photo diode is a main optical element of an optical receiver. Recently, a VCSEL diode is used as an optical transmitter in an optical transceiver. However, there are problems in that the VCSEL diode or a light emitting diode is highly temperature-dependent, threshold current is increased as temperatures rise, and current-light conversion efficiency is reduced. That is, when it is required to output a constant current from a drive circuit, optical output power after conversion is changed depending on temperature, and particularly under a high temperature condition, optical output power is reduced. Further, as light does not emit under the condition of a current below a threshold current, duty factor is degraded if a bias current (L level of an output current) is below the threshold current of the laser diode.
Accordingly, various methods have been used to stabilize communication characteristics by output of constant optical output power, including a method of feed-forward control in which temperature is determined using a thermistor, etc. so that an electric current value is dependent on a predetermined temperature, or a method of feedback control in which output of a laser diode is monitored with a monitoring photo diode (MPD) so that a constant electric current flows into the MPD.
More recently, optical transmission is conducted using a vertical-cavity surface emitting laser (VCSEL), instead of a laser diode. The VCSEL is an optical element, which is highly reliable with high speed operation and large-scale array arrangement, thereby enabling mass production and cost reduction.
Characteristics of the VCSEL are changed according to time and temperatures. For example, with respect to time, the VCSEL operates in such a peculiar manner that output power is gradually reduced until a certain threshold temperature, but beyond a certain threshold temperature, optical output power is gradually increased. A luminous element, such as LD and LED, may be temperature-compensated, but in a case a VCSEL is used as a luminous element, temperature characteristics unique to the VCSEL are difficult to be compensated. In optical transmission, temperature compensation is required in a wide temperature range of 0° C. to 85° C., and all temperature characteristics of the VCSEL are required to be compensated.